1. Field of the Invention
The present invention generally relates to a sequence instruction display system used with a sequence controller for displaying a sequence instruction and the on-off state of the corresponding input or output element.
2. Description of the Prior Art
A sequence controller automatically controls a controlled-objective such as a machine tool in accordance with a sequence program, which includes a large number of sequence instructions. The prior art display system is capable of displaying desired sequence blocks, each of which is made up on one output element and several input elements, and further displaying the on-off state of the elements. Such a system is disclosed in U.S. Pat. No. 3,964,026 to Nobuharu Yamauchi et, al. Using the prior art display system, an operator can check a sequence block when a machine trouble has occurred or, in other words, when an output element has not been energized. However, it is sometimes difficult to determine the cause of the machine trouble or an input element which concerns the energization of the output element, because an input element which is included in a sequence block may act as an output element in another sequence block, and therefore, the on-off state of such an input element may depend on the on-off states of other elements.
For this reason, it is necessary to check many sequence blocks, one after another, which concern the energization of the output element. However, when the prior art display system is used, the operator, after checking a sequence block, is required to repeat the previous manipulation, that is, the designation of the address data, for displaying a related sequence block. He must memorize the address data or look up the sequence circuit. For example, in the case of the sequence blocks shown in FIGS. 4 and 5, wherein an input element C12 in a first sequence block acts as an output element 02 in a second sequence block, if an output element 01 is not energized because of the off state of the input element C12, the operator has to designate the address data 150 of the output element 02 in order to display the second sequence block and to check which one of input elements C21, C22 and C23 prevents the energization of the output element 02 and further the output element 01.
Therefore, it is desirable to simplify the manipulation for displaying a desired sequence block or a sequence instruction.